A number of communication systems have been proposed for allowing the location of objects to be determined. Typically, a transmitter is co-located with an object whose location is to be determined. Signals from the transmitters are received by receivers which are installed at known locations. By measuring one or more characteristics of the communications between the transmitters and receivers the locations of the transmitters relative to the receivers can be estimated. In some systems the direction of communication is reversed: the transmitters are installed at fixed locations and the receivers are co-located with the objects. In others fixed transceivers are installed at the fixed locations and mobile transceivers are co-located with the objects, and bidirectional communication between the fixed and the mobile transceivers is possible. It is possible for the transmitter and the receivers to move, provided one set do so in a predictable way. For instance in the Global Positioning System (GPS) the locations of the satellite transmitters varies in a way that is known to the receivers.
When the objects that are to be located are within buildings there can be severe multipath effects which can hinder the accurate determination of the objects' locations. Because of this, ultra-wideband (UWB) radio is particularly promising for object positioning systems. By using short-pulse electromagnetic signals, UWB radio avoids many of the problems associated with conventional in-building radio positioning technology.
UWB communications systems using trains of pulses are discussed in the following papers: Multiple Access with Time-Hopping Impulse Modulation, R. A. Scholtz Invited Paper, IEEE MILCOM'93, Boston, Mass., Oct. 11-14, 1993 (http://www.timedomain.com/Files/downloads/techpapers/Sholtz.pdf); PulsON Technology Overview, A. Petroff and P. Withington, Time Domain Corporation. (http://www.timedomain.com/Files/downloads/techpapers/PulsONOverview7—01.pdf).
Numerous types of UWB radio systems exist. Several of them involve receivers at known locations that receive trains of low-power radio pulses sent by transmitters which are each co-located with an object whose location is to be determined. The receivers then integrate the energy in several pulses to recover the incoming signal. A difficulty of such systems is that there must be very tight synchronisation between the transmitters and the receivers since if the receiver is to recover the maximum incoming signal energy (and hence recover any incoming data with maximum reliability) its expected frequency of pulse arrival must match exactly the transmitter's frequency of pulse dispatch. This is illustrated in FIG. 1. FIG. 1 illustrates a train of pulses being sent from a transmitter to a receiver. If the received pulses are sampled at the correct frequency synchronisation then a strong impulse is detected. If the received pulses are sampled at an offset frequency synchronisation then a weak impulse is detected. Similar considerations apply in fields other than UWB radio systems.
In a typical UWB locationing system in which transmitters (tags) are located at the objects to be located and receivers (base stations) are installed at known locations, it could be anticipated that the clocks controlling the tag and base station must generate the same frequency to an accuracy of plus or minus around 1 ppm (parts-per-million) for reasonable performance. This figure would be a total frequency error budget taking both ends of the link into account. Thus, for example, if the base station had a theoretically perfect clock which operated exactly at the nominal pulse repetition frequency (PRF), the clocks on the tag could drift +/−1 ppm in frequency from the nominal PRF. Alternatively, base station clocks and tag clocks could both be permitted to drift up to +/−0.5 ppm from the nominal clock frequency, the worst case then being when a tag with a +0.5 ppm clock is communicating with a base station with a −0.5 ppm clock, or when a tag with a −0.5 ppm clock is communicating with a base station with a +0.5 ppm clock, but even these cases are within the anticipated frequency error budget.
Tags and base stations will typically have an on-board frequency reference oscillator, normally a quartz oscillator, acting as a clock for its transmissions and its reception operations. The accuracy of such an oscillator is generally affected by a number of factors:                initial manufacturing tolerance        manufacturing effects (e.g. extreme temperature during soldering)        temperature during operation        crystal ageing        
Some of these effects can be compensated for after the tag and base station devices have been manufactured. Each device can be compared with a ‘known, good’ frequency reference, and its frequency can then be tuned (e.g. by mechanical or electrical trimming) to bring it into synchronisation with the reference. This method can be used to compensate for any initial offset or any fixed offset introduced during manufacture. However, it cannot compensate for changes that occur during operation of the device, for example temperature-related effects and crystal ageing.
UWB positioning links differ from many standard communications links because they are typically is extremely sensitive to tag-infrastructure frequency offsets, which must be kept very small (typically less than ˜1 ppm) in order for the link to be established at all. Implementation of such systems therefore normally demands tag and infrastructure frequency references that are not greatly affected by temperature. As a result, the long-term (ageing) drift of the crystal is more problematic than the short-term (temperature) drift.
It might be thought that after compensating for initial fixed offsets, one could employ crystal references that did not age significantly, and that were relatively insensitive to changes in ambient temperature. However, in a practical commercial system power and cost constraints mean that the quality of oscillator that can be used is severely limited. In most applications the best oscillators that can be used for tags are temperature-compensated crystal oscillators (TCXOs), and for base stations oven compensated crystal oscillators (OCXOs). TCXOs typically have a best accuracy over temperature (in the range from 0 to 50° C.) of around +/−0.3 ppm, although +/−0.5 ppm TCXOs are significantly cheaper. A +/−0.3 ppm TCXO on base stations and a +/−0.5 ppm TCXO on the tags, both tuned after manufacture, would initially satisfy the +/−1 ppm total frequency budget. OCXOs are more expensive and power hungry, but have an accuracy over temperature of better than +/−0.1 ppm, and if installed at base stations would give more margin on the frequency budget.
However, ageing effects will be expected to change the frequency of a TCXO over the first year by +/−1 ppm, and over three years the frequency shift can be up to +/−3 ppm. An OCXO will be expected to age by up to +/−0.5 ppm per year. It therefore appears that in order to make such a communication system work over long periods of time without high maintenance overheads or excessively extensive components it will be necessary to compensate for the effects of crystal ageing in the tags and base stations.
Frequency control techniques are well-known in traditional radio communications systems. In particular, some narrowband FM systems provide an automatic frequency control (AFC) mechanism to allow a receiver to modify its operating frequency to match a transmitter which may have drifted off-centre. The Chipcon CC1020 FM transceiver manufactured by Chipcon AS (www.chipcon.com) is an example of a radio communications device with this capability. However, frequency adjustment schemes of this type are generally intended to compensate for short-term drift due to temperature changes, and so the basic mechanisms normally provide only for frequency optimisation of a link that has already been established. In contrast, the present invention relates to techniques suitable for use in establishing new links. According to preferred aspects of the present invention, measurements taken whilst that link is in operation can then be used to compute compensation values which will be stored and used for the next communication attempt.